CMP polishing slurry dewatering and reconstitution

ABSTRACT

A dry particulate solids composition is provided which may be reconstituted into a chemical-mechanical polishing slurry.

[0001] This application is a divisional of Ser. No. 09/413,083 filedOct. 6, 1999 which claims the benefit of U.S. Provisional ApplicationSer. No. 60/103,283 filed Oct. 6, 1998.

BACKGROUND OF THE INVENTION

[0002] Polishing formulations for the chemical mechanical planarization(CMP) of integrated circuit wafers, as well as polishing compounds forhigh technology optical components typically consist of an aqueousdispersion of solids (the abrasives) combined with a variety of chemicalconstituents. Such commercial materials typically are sold in severalparts, one containing the abrasive component in a concentrated form andthe other part containing the chemical component(s) in a concentratedform. As the technology of CMP polishing has become more critical,complex chemical and abrasive systems have been developed. These aqueousformulations must exhibit long shelf life, and good stability so thatthe materials do not change while in storage, thus becoming useless fortheir intended purpose. For example, if a dispersion of solids is storedand the solids settle in the container to a hard or difficult to mixsediment, the dispersion is no longer readily usable by the customer.

[0003] Many attempts to make one component systems in which the chemicalas well as the abrasive components are mixed in one liquid system haveproven problematic. The problems occur because the presence of thechemical constituents is rarely conducive to forming stable soliddispersions, consequently the solids settle to the bottom of thecontainer and require subsequent redispersion prior to use. Thispresents many problems to the end user. If the solids have settled intoa difficult to redisperse sludge, the chances of utilizing all of thesettled material are not good because it will stick to the container.Furthermore, shaking or stirring a large container such as a drum ortote is impractical.

[0004] The ideal concept would be to make a slurry exactly as the finalend user would use it, and then “freeze it in time” so that it will beexactly as the end user wants it, three or even six months after theslurry is made. Since the main reason for instability is due to theaqueous dispersions that these slurries are sold as, it would bereasonable to assume that if one could eliminate the “aqueous” part ofthe system, the stability issues for both the chemical and abrasiveconstituents would be minimized.

SUMMARY OF THE INVENTION

[0005] A dry particulate solids composition comprised of achemical-mechanical polishing slurry comprising submicron abrasiveparticles is provided which has had substantially all water removedtherefrom and which may be reconstituted into a chemical-mechanicalpolishing slurry ready for use.

[0006] A method for chemical-mechanical polishing is provided comprisingthe steps of:

[0007] a) providing a polishing pad;

[0008] b) introducing a polishing fluid comprising submicron abrasiveparticles between said pad and a workpiece;

[0009] c) producing relative motion between said pad and said workpiece;

[0010] wherein said polishing fluid has been reconstituted from a dryparticulate solids composition comprised of a chemical-mechanicalpolishing slurry comprising submicron abrasive particles which has hadsubstantially all water removed therefrom.

[0011] In addition to submicron abrasive particles, the dry particulatesolids composition of this invention may comprise an oxidizing agent, acomplexing agent, a surface passivating agent, a surfactant, adispersant, or any other type of compound used in slurries forchemical-mechanical polishing.

BRIEF DESCRIPTION OF DRAWINGS

[0012]FIG. 1 is a schematic of a typical spray dryer.

[0013]FIG. 2 is a schematic of a typical slurry reconstitution.

[0014]FIG. 3 is a comparison of particle size distribution for threeslurries of Example 1.

DESCRIPTION OF THE INVENTION

[0015] This invention provides a means of preserving the exact abrasiveand chemical ratios without the destabilizing and aging effects ofaqueous dispersions. The abrasive and chemicals are mixed together as ifthey were being prepared for the end user's immediate use, or they areprepared as a concentrate with the ratios of the components being allrelated by a common multiple. Immediately after the polishing slurry ismanufactured, it is dewatered by spray drying, freeze drying, or anynumber of drying or dewatering methods that exist. The resulting driedmaterial, usually in the form of a flowable powder, can be packaged inconventional bags and stored indefinitely without degradation.

[0016] A further advantage brought about by this invention is that theweight of the product is reduced by as much as 70%-80%, which is aconsiderable cost savings when one considers shipment of liquid, aqueousslurry worldwide.

[0017] When the slurry is ready to be used, the end user mayreconstitute the dried material by adding the required amount of waterand dispersing with a high shear disperser. The resulting slurry isfiltered and is ready for use in the CMP polishing operation.

[0018] The resulting polishing slurry does not acquire unwantedaggregates as one would expect. This is thought to be due to thecomplete and thorough dispersed state that the slurry is brought to justprior to drying. As the colloidially dispersed mixture of componentsdries, the abrasive grains are surrounded by the uniform presence of thedrying dissolved salts, which in effect protect the abrasive particlesfrom agglomerating with each other. As the salts are solvated during thereconstitution process, the abrasive particles are released inessentially the same state they existed prior to being dried.

[0019] Typical submicron abrasives used in chemical-mechanical polishingslurries are oxides such as alumina, silica, ceria, titania, germania,zirconia, and the like. Generally abrasive particles are used inslurries for CMP at about 1% to about 15% by weight. Preferred arealumina, silica, ceria, titania, or mixtures thereof at about 3% to 10%by weight.

[0020] Types of chemicals useful in CMP operations are oxidizing agents,chemical etchants, dispersing agents, surfactants, complexing agents,silica rate suppressing agents, passivating agents, silica protectingagents, buffers, and inhibitors, all of which may be present in thedried slurries of the present invention.

[0021] Commonly hydroxides, such as potassium hydroxide, ammoniumhydroxide, and sodium hydroxide, and amines have been used as dispersingagents for CMP slurry abrasives. It has been found that a class ofcompounds known as amino alcohols may also be useful.

[0022] An oxidizing agent is usually a component of achemical-mechanical polishing slurry to oxidize a metal layer to itscorresponding oxide, such as oxidizing tungsten to tungsten oxide. Thelayer is mechanically polished to remove the tungsten oxide from thelayer. Although a wide rnage of oxidizing components may be used,preferred components include oxidizing metal salts, oxidizing metalcomplexes, iron salts such as nitrates, sulfates, EDTA, citrates,potassium ferricyanide and the like, aluminum salts, sodium salts,potassium salts, ammonium salts, quaternary ammonium salts, phosphoniumsalts, peroxides, chlorates, perchlorates, permanganates, persulfates,iodates, and mixtures thereof. Typically, the oxidizing component ispresent in the slurry in an amount sufficient to ensure rapid oxidationof a metal layer while balancing the mechanical and chemical polishingcomponents of the slurry. Oxidizing agents are typically present in achemical-mechanical slurry from about 0.5% to 15% by weight, andpreferably in a range from about 1% to 7% by weight.

[0023] Compositions of this invention may optionally further comprisecompounds which act as complexing agents or chelating agents for SiO₂.These are described in great detail in U.S. Pat. No. 5,391,258 and U.S.Pat. No. 5,476,606. These compounds must have at least two acid groupspresent in the structure which can affect complexation to the silica.Acid species are defined as those functional groups having a dissociableproton. These include, but are not limited to, carboxyl, hydroxyl, sulfoand phospho groups. Carboxyl and hydroxyl groups are preferred as theseare present in the widest variety of effective species. Particularlyeffective are structures which possess two or more carboxyl groups withhydroxyl groups in an alpha position, such as straight chain mono- anddi-carboxylic acids and salts including, for example, malic acid andmalates, tartaric acid and tartarates and gluconic acid and gluconates.Also effective are tri- and polycarboxylic acids and salts withsecondary or tertiary hydroxyl groups in an alpha position relative to acarboxyl group such as citric acid and citrates. Also effective arecompounds containing a benzene ring such as ortho di- andpolyhydroxybenzoic acids and acid salts, phthalic acid and acid salts,pyrocatecol, pyrogallol, gallic acid and gallates and tannic acid andtannates. These complexing agents may be used in slurries for CMP atabout 0.1% to about 7% by weight. Preferably they are used at about 2%to about 4% by weight.

[0024] In addition the slurry composition may include a chemical etchantwhich is substantially free of metal ions. Typical etchants includepersulfate salts, nitrate salts, sulfate salts, phosphate salts, citratesalts, oxalate salts, mixtures thereof, and the like. Preferably, theetchant is a non-metallic persulfate salt such as ammonium persulfate.The etchant facilitates the solubilization of the metal where thechemical mechanical polishing is taking place, thus allowing the metalto be dissolved in the aqueous dispersion. These chemicals are generallyfound in CMP slurries at about 1% to 10% by weight. Preferably they areused at about 2% to 7% by weight.

[0025] Often a chemical-mechanical polishing slurry will comprise acorrosion inhibitor which is substantially free of metal ions. Suitablecorrosion inhibitors include benzotriazole (BTA), mercaptobenzothiazole(MBT), and other corrosion inhibitors typically used with metals such ascopper which do not adversely affect the properties of the slurry.Typically corrosion inhibitors are present in chemical-mechanicalpolishing slurries at less than about 1% by weight. Preferably they arepresent in the range from about 0.05% to 0.6% by weight.

[0026] The first step in preparing dried slurry materials is todetermine the exact ratio of all of the components in a polishing slurryas they exist at the time of use. Let us assume that the slurry we wishto reconstitute is a 5% solid abrasive and 7% chemical constituents, andthat there are two chemical constituents. The total composition of aready to use formulation may look similar to the following example:Solid Abrasive 5% Chemical A 3% Chemical B 4% Water 88% 

[0027] After the water has been removed, the solids and dissolved solidswill be left in the ratio of 5/3/4. In spray drying it is generally goodto have the highest solids content possible, to reduce the amount ofwater that has to be evaporated, thus reducing the cost and increasingthe throughput. Thus, when making a concentrate for drying, one shoulduse the constituents in the ratio of 5/3/4 times some factor. Thisfactor is typically determined by a solubility limit of one of thecomponents, or a maximum solids loading limitation. Let us assume thatChemical B is only soluble to the maximum extent of 10%, then in orderto make a completely homogeneous dispersion where all of the chemicalcomponents are dissolved, the maximum concentration factor would be10%/4% or 2.5. Thus, the concentration of the components of thepre-dried slurry is determined by multiplying each component except thewater by the factor 2.5. The final concentration for the drying slurryis: Solid Abrasive 2.5 × 5 = 12.5% Chemical A 2.5 × 3 = 7.5%  Chemical B2.5 × 4 = 10.0% Water Balance 70.0%

[0028] This formulation when dried will yield a dried material with thethree active constituents having the ratio of 5/3/4 as in the ready touse slurry but by concentrating the spray dryer starting material, thespray drying operation is more efficient.

[0029] The drying technique can be one of many including spray drying,freeze drying, flash drying, vacuum drying, heated pan or conveyordrying, etc. The preferred one chosen for this invention is spray dryingbecause it offers the most easily controlled final product with respectto particle size, % moisture, and reproducibility of product.

[0030] A spray dryer utilizes the combination of hot air circulating ina large tank into which a solid/liquid slurry is sprayed, either byrotary atomization or high pressure aspiration, into the heated airwherein the water content is almost immediately removed and a water-freesolid particle is formed. As the particle progresses further in the airstream, the particle is further dried until it reaches its exit point inthe chamber and is captured by a cyclone or collection container. Aschematic of a typical spray dryer is shown in FIG. 1. In many dryersthe coarser material is captured in a product collection container andthe fines are collected in a cyclone separator. In the case of thisinvention, both collection methods are satisfactory as the size of thedried particle is not an issue.

[0031] The dried product is then transported to a storage hopper whereinenough spray dried material is collected to make a full lot. Thecollected product is then dry blended in a large double cone blender, orsimilar type, so that the dried powder lot is completely homogeneous. Byblending a large lot dry, the size of the individual blended lot can bemuch larger than its liquid equivalent because the weight and volume ofthe water has been removed from the concentrate. A large homogeneous lotsize is particularly important for the semiconductor industry, whereeach new lot of material must be checked prior to use.

[0032] After blending, the powder is packaged in any suitable type ofcontainer, such as a plastic lined fiber drum, or a plastic lined paperbag. It is desirable for the container size to be equivalent to theamount of dried material that is likely to be used in each polishingcampaign after reconstitution.

[0033] Reconstitution of the dried material may be carried out in amixing unit as shown in FIG. 2. The equipment consists of a suitablemeans of introducing the measured amount of water and the powderedslurry. The mixer is a high speed rotor stator design capable ofproviding very high shear. After the reconstituted product has beenmixed adequately (generally determined by how much energy per gallon hasbeen expended on the product) the product is pumped from thereconstituting equipment through a submicron filter system and into aday tank. From the day tank, the product is recirculated and used by theend user in his polishing application.

[0034] It is in this last operation that the novelty and benefit of thisinvention is demonstrated. If the slurry is a two component system, thecomponent containing the solids typically must be premixed toredistribute the solids in the container prior to dispensing into theday tank. If this is not done, the correct % solids in the final mixturewill be incorrect. Similarly, the second component should be pre-mixedto homogenize the solution prior to adding to the day tank. Both ofthese components generally require some type of metering, either byweight or volume, in addition to the metering of water in someconcentrated systems. This mixture must then be filtered to remove anycontamination from the operation.

[0035] In the case of this invention, no pre-mixing is required, becauseno settling or stability issues are involved. The powder is merelyweighed (or pre-weighed for each size day tank), poured or educted intoa tank containing the correct volume of water, and mixed. Uponcompletion of mixing, the final product is pumped through a filter tothe day tank. In some cases, the day tank could be utilized as themixing and dispersing vessel.

EXAMPLE 1

[0036] A slurry formulated for polishing copper in integrated circuitapplications, Met410, was used as one example of this invention. Thisslurry typically has a very short shelf life as a one component slurry,and only a short (several weeks) as a two component slurry becauseseveral of the components react with one another. Another component islight sensitive in the liquid form. The drying of this slurry is furthercomplicated by the fact that several of the salts are ammonia salts andeasily decomposed by heat.

[0037] The Met410 slurry was prepared as a 2× concentrate to facilitatethe spray drying operation. This represents a slurry that is twice asconcentrated in each of the solids and dissolved solids as a slurrynormally used for polishing. The ingredients of this predrying slurrywere as follows: Phosphoric acid: 11.2%  Ammonium Phosphate 8.5%Ammonium persulfate: 8.5% Organic corrosion inhibitor 0.3% Titaniumdioxide 9.9% D.I. water Balance

[0038] The ingredients were mixed together and dispersed with a Hillrotor stator mixer until the temperature rise in the tank reached 10degrees Celsius.

[0039] Within three days a portion of this material was spray dried inan APV Laboratory Spray Dryer having an electric air heater, a rotaryatomizer, and a peristaltic feed pump with variable speed motor. SeeFIG. 1. The dried powder was collected from the bottom of the dryer aswell as the cyclone separator. The atomizer was adjusted to preventproduct from sticking on the walls of the dryer, and the inlet airtemperature and feed rate were adjusted to obtain an outlet air temp of80-85 degrees Celsius. The powder was sealed in a plastic bag awaitingfurther processing. The remaining slurry was stored in a closedcontainer as a liquid awaiting further testing. The details of thisspray drying test are listed in Table 1 below: TABLE 1 Inlet Temp.Outlet Temp. Feed Rate Atomizer Test (° C.) (° C.) (mL/min) speed(rpm) I250-260 80-85 130 49000 II 180-185 80-85  65 41000

[0040] The two tests denote two different inlet temperatures at whichthe dryer was operated to determine if inlet temperature had an effecton the decomposition of volatile or heat sensitive components.

[0041] Another means of monitoring the usefulness of this invention isin the analytical determination of the primary active ingredient in theslurry, the ammonium persulfate. This component is both sensitive toheat and to light in the aqueous state. The two dried slurries and theconcentrate from which they came were analyzed one week after the spraydrying was performed. The results are shown in Table 2 below. Allconcentrations were made in the ready to use concentration.

[0042] From Table 2 it can be seen that the spray drying preserves theefficacy of the least stable chemical component. Even within ten days ofits manufacture, the strength of the oxidizing component, ammoniumpersulfate, was decreasing within the single part slurry concentrate.TABLE 2 median mean Slurry % persulfate pH particle size particle sizeReconstituted test I 4.34 2.3 0.178 0.233 Diluted liquid con- 4.17 2.30.177 0.225 Centrate, 10 d. old Reconstituted test II 4.33 2.3 0.1780.227

[0043] It can be seen from the data in Table 2 that the persulfateconcentration did not change with increased inlet temperature andremained exactly as it was intended in the original composition. Theparticle size of the two dried materials came back to exactly what theyhad been before the drying operation. The graphical depiction of thecomparison of particle size distributions of the predrying concentrate,and the two reconstituted test slurries is shown in FIG. 3. The threematerials are so close that the three distributions appear to be one.

[0044] Six weeks after spray drying, polishing tests were performed onthe pre-drying slurry concentrate and the dried slurry sample Test I ofthe Met410 slurry. The final concentrations of these slurries wereadjusted to the following levels: Phosphoric acid: 5.6% AmmoniumPhosphate 4.3% Ammonium persulfate: 4.3% Organic corrosion inhibitor 0.15% Titanium dioxide 4.9% D.I. water Balance

[0045] The spray dried slurry was measured out to obtain the aboveconcentration and dispersed in a Hill rotor stator mixer to a delta T of10 degrees Celsius. The residual six week old concentrated slurry wasdiluted approximately 1 part concentrate to 1 part water to get theexact concentration noted above and mixed with a propeller mixer. Allslurries were filtered through a 1 micron bag filter.

[0046] Six inch diameter copper film wafers were polished with each ofthe three slurries using a Strasbaugh 6EC Wafer Polisher at identicalpolishing parameters. Also polished were silicon dioxide film wafers toestablish the relative polishing rate between copper and silicon dioxideknown as the oxide selectivity. The results of these tests are shown inTable 3. The polishing rates are listed in Angstroms per minute (A/m):TABLE 3 Slurry Origin Copper Removal Rate Selectivity to Oxide Predryingconcentrate, 2200 A/min 26:1 Cu:Oxide rate diluted to use conc. Spraydried reconstituted 7049 A/min 63:1 Cu:Oxide rate slurry # I

[0047] Typical removal rates for Met410 slurry are 6000 to 7000 A/min.The six week old concentrate had deteriorated considerably during thattime as a one component mixture. The spray dried material had preservedthe characteristics of the commercial two part slurry.

EXAMPLE 2

[0048] The second spray dried material known as Test II above was testedin a copper wafer polishing comparison nine weeks after it was dried.The test was run as a comparison of the reconstituted Test II materialand a freshly mixed Met410 slurry. The polishing was performed inessentially the same manner as before. The concentrations of the twoslurries were made up to the end use concentration shown above. Resultsof the polishing tests are shown in Table 4 below: TABLE 4 Slurry OriginCopper Removal Rate Selectivity to Oxide Fresh Met410 slurry 6100 A/min36:1 Cu:Oxide rate Spray dried reconstituted 6600 A/min 66:1 Cu:Oxiderate slurry # II

[0049] The surface finish of these wafers was also studied. Thereappeared to be no noticeable difference in surface scratching betweenthe two slurries.

EXAMPLE 3

[0050] A concentrate identical to the pre-drying concentrate of examples1 and 2 was prepared in the same manner in preparation for another spraydryer test.

[0051] Within three days a portion of this material was spray dried inan Niro Production Minor Spray Dryer having an electric air heater, arotary atomizer, and a Moyno feed pump with a variable speed motor. Allof the dried powder was collected from the cyclone separator. Theatomizer was adjusted to prevent product from sticking on the walls ofthe dryer, and the inlet air temperature and feed rate were adjusted toobtain an outlet air temp of 100 degrees Celsius. The powder was sealedin a plastic bag awaiting further processing. The remaining slurry wasstored in a closed container as a liquid awaiting further testing.

[0052] Several weeks later polishing tests were performed on thepre-drying concentrate slurry of example 3 and dried slurry samples aswell as a commercial two part version of the Met410 slurry. The finalconcentrations of all three slurries were adjusted to the followinglevels: Phosphoric acid: 5.6% Ammonium Phosphate 4.3% Ammoniumpersulfate: 4.3% Organic corrosion inhibitor  0.15% Titanium dioxide4.9% D.I. water Balance

[0053] The spray dried slurry was measured out to obtain the aboveconcentration and dispersed in a Hill rotor stator mixer to a delta T of10 degrees Celsius. The residual concentrated slurry from the spraydrying tests was diluted approximately 1 part concentrate to 1 partwater to get the exact concentration noted above and mixed with apropeller mixer. The commercial slurry was made by mixing Part A andPart B together (to obtain the above concentrations) and mixing with apropeller mixer. All slurries were filtered through a 1 micron bagfilter.

[0054] Six inch diameter copper film wafers were polished with each ofthe three slurries using a Strasbaugh 6EC Wafer Polisher at identicalpolishing parameters. The results of these tests are shown in Table 5.TABLE 5 Slurry Origin Copper Removal Rate Fresh Met410 slurry 6960 A/minSpray dried reconstituted slurry from 6333 A/min Niro Tests Predryingconcentrate, diluted to use 4994 A/min concentration (2 wks old)

[0055] Microscopic examination of these test wafers showed that thesurface condition of the freshly made commercial slurry was better thanthe spray dried and reconstituted slurry in this case. The reason forthis discrepancy is unknown.

EXAMPLE 4

[0056] A Niro Mobile Minor Type H spray dryer was used in this example.This dryer was operated in a similar manner to the other examples. Thepolishing slurry in this example is used for polishing tungsten. Theproprietary slurry s a ready to use formulation had the followingconstituents: Carboxylic acid 3% Carboxylic acid salt 0.3%   Oxidizingcomponent 6% Abrasive 9% D.I. Water Balance

[0057] A concentrate of this slurry was prepared for spray drying bydoubling the concentrations of the chemical and abrasive components. Theconcentrate was then spray dried at several outlet temperature values todetermine the degradation, if any, of the carboxylic acid components.The outlet temperature was varied by changing the feed rate of theconcentrate and leaving all other parameters constant.

[0058] The dry flowable powders were reconstituted to the ready to useslurry composition shown above, and used to polish tungsten film wafersand thermal oxide film wafers. A portion of the original concentrateused to make the powders was also diluted to the ready-to-useconcentration and used as the baseline. Table 6 below shows the resultsof the spray drying and polishing tests: TABLE 6 Tungsten rateSelectivity to ox. Diluted concentrate from spray 2344 A/min 416:1drying run Freshly made slurry 2177 A/min 386:1 Reconstituted spraydried 2328 A/min 346:1 powder

[0059] Microscopic examination of the surfaces of the oxide film wafersafter polishing showed no difference in scratching between any of thetest wafers.

[0060] Although the examples above report the use of this invention withchemical mechanical polishing slurries for integrated surfaceapplications, the value of this invention in other types of polishingformulations is obvious also. For example, many polishing compoundscontaining abrasives and chemicals, added to provide enhanced removalrates or special surface conditions, could be dried in a similar mannerand reconstituted at the point of use. Abrasive slurries such as thisare used for polishing specialty optics, for polishes involvingsemiconductor substrates such as silicon and gallium arsenide, forplastic eyeglass and contact lenses and other similar technologies.

1. A dry particulate solids composition comprised of achemical-mechanical polishing slurry comprising submicron abrasiveparticles which has had substantially all water removed therefrom.
 2. Acomposition according to claim 1 wherein said submicron abrasiveparticles are from the group comprised of alumina, silica, ceria,titania, and mixtures thereof.
 3. A composition according to claim 2wherein said submicron abrasive particles are titania.
 4. A compositionaccording to claim 1 further comprising an oxidizing agent.
 5. Acomposition according to claim 1 further comprising a chemical etchant.6. A composition according to claim 1 further comprising a dispersingagent.
 7. A composition according to claim 1 further comprising acomplexing agent.
 8. A composition according to claim 1 furthercomprising a corrosion inhibitor.